Genetic and hereditary influences that give rise to variation in the magnitude and direction of lateralization are presently the least understood processes in the etiology of lateral specialization and cerebral dominance. Because mice exhibit marked asymmetries of behavior that are analogous, if not prototypic to the expressed functional asymmetries of human beings, and because the knowledge of the genetics of the mouse is far more advanced than that of any other mammal, studies of the hereditary bases of laterality in mice can make unique contributions to an understanding of lateralization of function and cerebral dominance. The objective of this research is to gain an understanding of the genetic, hereditary, neurobehavioral, and developmental processes leading to variability in the expression of lateral specialization in genetically defined and definable strains and stocks of mice. The long-term goal is to create and characterize practical experimental models of variable lateralization that may prove useful for the study of specific behavioral differences, deficits, and associated psychopathology that may be associated with variability of lateralization in humans. Mice of standardized inbred strains are the primary subjects of this research. Directional selection for the degree of magnitude of lateralization for handedness has been carried out for 11 generations. Selected lines diverged at the third generation of selection and were maximally different at the 10th generation. The lines are being maintained by random mating for studies of associated characters. In addition, a series of 50 incipient inbred strains lines is being developed to provide a unique mammalian resource for studies of the complex genetic-environment interactions that culminate in the expression of the directed senses of neurological asymmetries. The selected lines and substrains are also a promising resource for the isolation of regulatory and structural genes at complex loci.